Refractory compositions



United States Patent 3,251,700 REFRACTORY COMPOSITION Victor Mandorf, Jr., Fostoria, Ohio, assignor to Union Carbide Corporation, a corporation of New York No Drawing. Filed June 24, 1963, Ser. No. 290,236

- 5 Claims. (Cl. 106-65) This invention is directed to novel refractory compositions which are chemically and thermally stable and more particularly to compositions comprising aluminum nitride as an essential constituent The compositions of the present invention are chemi cally and thermally stable and have certain outstanding properties which make them extremely desirable for use in many fields. As used herein, chemically and thermally stable means that the compositions are more resistant, in general, to chemical change and physical deterioration induced by heat than ordinary materials.-

To be useful as refractory materials a composition must be able to withstand exposure to high temperature for extended periods of time without serious chemical and physical change. Such materials should be able to resist cracking or deterioration due to sudden changes in temperature. Relatively high mechanical strength and resistance to corrosion and oxidation over a wide range of temperatures are other properties which are desirable inrefractory materials.

It is an object of this invention to provide refractory compositions which exhibit a high degree of resistance to oxidation at elevated temperatures.

It is another object to provide refractory compositions having a thermal shock resistance greater than that of pure titanium diboride.

The present invention contemplates a composition comprising aluminum nitride in combination with titanium diboride.

Such compositions are found to exhibit superior resistance to thermal deterioration induced by sudden and substantial changes in temperature and further, to be extremely resistant to corrosion and oxidation. Moreover, articles fabricated from such compositions are found to have other outstanding properties such as high strength and controlled electrical resistivity. The electrical resistivity of the compositions can be varied over a wide range by varying the relative amounts of aluminum nitride and TiB In general the present invention contemplates a composition comprising from 1 to 99 weight percent aluminum nitride, the remainder being TiB A preferred composition comprises from about 25 to about 85 weight percent aluminum nitride, inclusive, and

from 15 to 75 weight percent titanium diboride.

The presence in the compositions of the invention of up to 40 weight percent aluminum nitride increases the thermal shock resistance of articles formed from the compositions, without causing changes in electrical conductivity. The presence of aluminum nitride in amounts in excess of 75 weight percent in the compositions of the invention results in substantial changes in electrical resistivity of the formed article. By controlling the aluminum nitride content from about 75 to about 85 weight percent, the electrical resistivity can be varied from about 10 to about 10 micro-ohms-centimeters. Such articles are useful as high temperature resistors since they exihibit outstanding resistance. to both chemical change and physical deterioration at temperatures above 1000 C.

"ice

Table I illustrates the variation in electral resistivity of titanium diboride-aluminum nitride compositions with increasing concentration of aluminum nitride.

The aluminum nitride-titanium diboride compositions described herein'have been found to exhibit a resistance to thermal shock which is superior to that of pure titanium diboride. Many ceramic and refractory materials exhibits a substantial amount of high temperature strength. Unfortunately however, many such materials do not provide an adequate degree of resistance to thermal shock and are therefore unsatisfactory for certain high temperature applications. The requirements of thermal shock resistance are particularly important when the refractory material is required to provide service over a long period of time while being cycled from hot to cold, as for example, in the case of electrical ignitors.

The properties of refractories which in varying degrees influence the resistance to thermal shock are thermal expansion, elastic modulus, and strength, and thermal conductivity. Generally, high values of flexural strength and thermal conductivity combined with low values of elastic modulus and coefiicient of thermal expansion can be expected to indicate an excellent resistance to fracture caused by thermal shock. The addition of aluminum nitride has been found to substantially increase the thermal shock resistance of a refractory material, i.e., titanium biboride.

Table H shows the advantages achieved through the addition of 10' weight percent of aluminum nitride to titanium diboride with respect to improving the thermal shock resistance. For these tests a series of samples were hot pressed in the form of rods having a diameter of 3 /2 inches and a length of 10 inches. The rods were heated inductively at a rate of 500 C. per hour to the temperatures indicated in column 2. The rods were then removed from the furnace and allowed to air cool. In each case, the rods containing aluminum nitride and those containing no aluminum nitride were allowed to cool under the same conditions. The rods containing 10 weight percent aluminum nitride remained unaffected until they were cooled from temperatures of 1000 C. When cracking did occur in these samples it was not as severe as that encountered in the pure titanium di'boride samples. These results indicate a considerable improvement in thermal shock resistance for the small amounts of aluminum nitride involved.

TABLE II Composition Tempera- Resultsture, 0.

Some slight cracking. N0 cracking.

Severe cracking.

No cracking.

Very severe cracking. Some cracking.

TiBg TiB +10AlN num nitride and titanium diboride, to form a homogeneous blend. These materials may be reduced to the Table IV is a tabulation of some physical properties of the sintered bars.

form of a powder by any convenient technique such as TABLE Iv ball milling and the like. In general, the particle size is not narrowly critical and may range from about 1 5 Percent of Flexuml micron to about 5 microns in diameter. The homoy Theoretical Resistivity8 Strength geneous admixture of particles can then be placed in a Densty mold and hot pressed in an inert atmosphere to form the 50 0 2 94 77 4 355 13 800 desired shape. 41:71:11: 3135 86:0 127 271000 The hot pressing operation is carried out under stand- 8911 89 27,800 ard conditions well known in the art, e.g., 1800 C. and 52 29'600 2000 pounds per square inch.

The hot pressed compositions thus produced are found 2235.1; giifgfi fiiig to have a relatively high percent of theoretical density. Hf i t D Densities to 96 percent of theoretical have been obtained 15 ounds per square me a 25 for aluminum nitride-titanium diboride compositions. Resistance to oxidation f aluminum nitride titanium Alternatlvfilya th homogeneous mixtures have been diboride compositions were determined by measuring the sintered to densities of from 70 to 95 percent of theoreti i Weight f one quarter inch diameter by two inch ical maximum density. The admixture 1s milled to a suitlong Samples upon exposure to at 10000 c flowing able p i S116, mlffed With a fugltlve binder and Cold through an oxidation furnace at a rate of about 3 cubic pressed into the desiredshape. These molded articles f t per helm The Weight changes were continuously. are then sintered an Inert atmcsphere at a p measured by an automatic gravimetric balance. A sammre between about 18000 21000 ple of 100 percent titanium diboride gained 0.0035 gram- The thfiofefical density as usefl herein, including per square centimeter after 7.5 hours, whereas a comthe pp Chum represents a dvenslty Calculated from position containing 75 weight percent aluminum nitride. h dffnslty 0f each component and the relative P P gainedonly 0.00134 gram per square centimeter after nous mvolved- 44 hours of exposure. Another sample containing 80 Example 1 weight percent aluminum nitride was found to have gained Approximately 680 grams aluminum nitride and 5120 0130105 gram P Square cniimetef afte? 3 Period 0f 60 grams of titanium diboride were blended in a ball mill hours- Further i1'1Ve$tig1t1 0I1 has imllcated that the for a period of 24 hours. The particles of titanium di Presence f aluminum nitride subst y fed}l ces the 1 b id h d b previously b ll i l d to reduce m rate of oxidation of tltanium diboride compositions ataverage particle size to from 2 to 2.5 microns. The filevatfifl temperatures homogeneous blend was then screened through a mesh cW 0f the above described p p the screen to break up any agglomerates, and loaded by 0 P lllventlon ll be advfmtageously P YF vibratory ki i hi ld h i an i id ias materials of construction for high temperature resisameter of 3 /2 inches. The blend was then hot pressed tors, g ceramic P9 and thtjllkein an argon atmosphere at 1800 C. and 2000 pounds For eXample, a Q P mp g 82 L Q per square inch pressure, in a resistance heated tube 40 pf f aluminum nltrlde f 18 Welght P tlfalllum furnace. The furnace was allowed to cool at its natural diboride Was hot Prfissed mm a rod havlng dlamfitel' rate while the mold was maintained under pressure. of one quarter inch- A {W0 inch Piece of this Tod Was Several compositions containing varying amounts of 1156(1 as an ignitof element ignitlng a g flamealuminum nitride and titanium diboride were prepared Such ignitof element Operated successfllllyfof 4,76;4 ignl" as above and some of their physical properties measured. tion cycles. During these tests the ignrtor attamed a The results of the measurements are tabulated in Table temperature of about 1200 C. almost instantly. The III. element continued to ignite the gas throughout the test TABLE III Flexural Strength, Tcmp., Pressure, Percent of Resisp.s.i. AlN C. p.s.i. Density Tlreor. tivity 3 Density at 25 C 1, 800 2, 000 3. s7 s0. 3 22. 5 1, 300 2, 000 3. 49 95. 0 400 30, 000 1,800 2,000 3. 35 95. 5 1000 as, 400 33,000 1, 800 2,000 3.28 95.0 5000 35, 040 31, 000 1,800 2,000 3. 27 95.5 2.3 10 1,800 2,000 3.14 92.5 5 0x10 29, 500 16,500

1 Weight percent of AlN. 2 Grams per cubic centimeter. 3 Micro-ohm-ccntimeters.

Exam le 2 and exhibit outstanding fiexural strength both before A blend of 750 grams of titanium diboride and 250 and after the test was completed. Arr examination of the grams of aluminum nitride were n mined to a element at the end of 4,764 cycles disclosed no sign of homogeneous admixture as described in Example 1 The cracking or deterioration due to thermal shock, thus e'viadmixture was then mixed i a Polyethylene oxide dencing the superiority of titanium diboride-aluminum fugitive binder and cold pressed at 20,000 pounds per nitride compositions over tungsten and silicon carbide square inch into rectangular bars /2" x /3" x 6". These 7 neither of which materials would withstand such treatbars were then sintered in an argon atmosphere at 2050 C.

Several additional compositions containing varying amounts of aluminum nitride in admixture with titanium ment.

What is claimed is:

1. A refractory composition consisting essentially of titanium diboride and from 1 to 99 weight percent alumi- .diboride were prepared and sintered as described above. num nitride.

6 2. A refractory composition consisting essentially of titanium dibon'de and from to 85 weight percent titanium diboride and from 25 to 85 weight percent of aluminum nitride said composition having a density of aluminum nitride. between and 96 percent of the theoretical density.

3. A refractory composition comprising from to weight percent aluminum nitride and the balance tita- 5 References Cited by the Examiner I nium diboride. UNITED STATES PATENTS 4. A refractory composition comprising from 10 to 2,929,126 3/1960 Bonack et 5 40 weight percent aluminum nitride and the balance 103 3 7 10 19 3 Lenie et 1 10 55 titanium diboride.

5. A refractory composition consisting essentially of 10 TOBIAS E. LEVOW, Primary Examiner. 

1. A REFRACTORY COMPOSITION CONSISTING ESSENTIALLY OF TITANIUM DIBORIDE AND FROM 1 TO 99 WEIGHT PERCENT ALUMINUM NITRIDE. 